Polymorphism #

Polymorphism (having many forms) is the ability of an entity in an object-oriented model to refer to objects of different classes at different times. For example, a method that accepts an object of a certain class will also operate on its subclasses:

public class MyShapeCollection {
  private List<Shape> shapes;

  public MyShapeCollection() { shapes = new ArrayList<>(); }
  public void add(Shape shape) { shapes.add(shape); }
  public double getTotalArea() {
    double total = 0;
    for (Shape shape: shapes) {
      total += shape.area();
    }
  }
}

We can pass any subclass of Shape to the add method:

MyShapeCollection myShapes = new MyShapeCollection();
myShapes.add(new Circle(2.0));
myShapes.add(new Rectangle(1.0, 3.0));
myShapes.add(new Rectangle(4.0, 2.0));
System.out.println(myShapes.getTotalArea());

We can store and iterate over any subclass of Shape in the shapes collection. When we invoke area() on a shape object, the polymorphic implementation of area() is selected at runtime based on what the "actual" (instantiated) type of the shape is (i.e., whether it is a Circle or Rectangle).